Help me select value brake replacement

I have an entry-level 29er that I am swapping out most trash components on a budget. It came with the dreaded Tektro Novela mechanical disc brakes and they are next on the chopping block. With a budget of $100, what would you reccommend and why? I am looking at these as options:

Budget and brake should not be in the same sentence. Buy the best you can with the money you have. In which case, to save the most trouble switching stuff around, I would go with BB7s if you can spring it, or BB5s if you can't. Both come with rotors standard. Both have good reliability, and both are well reviewed. Between the two, adjusting the BB7 will be easier, but both brakes have good stopping power, if money is a concern.

The avid levers have a few advantages (the SD-7s, specifically). They have an enormous adjustment barrel, at least 1/3 larger than most others (helps you get the right tension and keep it on the cable) a barrel for adjusting cable pull, and a reach adjustment nut. Between the three, you get unparalleled control over your lever feel and cable pull.

You should be fine with the rotors you have, though having a pair of backups would be nice.

BB7's bought from bike shops will come with rotors whether you want them or not.
If you want BB7's without rotors you'll probably find some on ebay, the brakes will be OEM version and they'll be located somewhere in Asia.

Are your current brake levers and shifters combined ?
If they are and you buy new brake levers you'll also need to get new shifters too.

Budget and brake should not be in the same sentence. Buy the best you can with the money you have. In which case, to save the most trouble switching stuff around, I would go with BB7s if you can spring it, or BB5s if you can't. Both come with rotors standard. Both have good reliability, and both are well reviewed. Between the two, adjusting the BB7 will be easier, but both brakes have good stopping power, if money is a concern.

Of the options you have listed in your first post, get the BB7's - even if you can't get the SD-7 levers, get the BB7's. They will work fine with other levers if you set them up right to start with. If you keep the inner and outer pad adjustments right, and your initial setup is correct, you'll never need the barrel adjuster at all, never mind a longer barrel adjuster. Don't get me wrong, the SD-7's are nice levers, but you need focus on the calipers for now it sounds like.

I appreciate the input from everyone, very well received. After reviewing the technology of each kit and the prices I went the cheapest route.

I got BB5's and they pull very tight. I have not yet got the levers because I installed the calipers well enough. My bike is also brand new so it has been in for a cable tightening and after that it is excellent. I got the pair of calipers, a 2012 set unused, for $45 shipped so I feel like I made the right decision. Throughout the upcoming season if things go differently I will definitely review your input and look at those above as possible replacements.